I don't know what happens here internally, but the problem goes away if we use Abs instead of Norm:
Reduce[{Abs[z1] == Abs[z2] == 2, z1 + z2 == Sqrt[3] + I,
y == Abs[z1 - z2]}, {y, z1, z2}, Complexes]
Solve[{Abs[z1] == Abs[z2] == 2, z1 + z2 == Sqrt[3] + I,
y == Abs[z1 - z2]}]